The present invention relates to a thermal transfer sheet, and more particularly relates to an improvement in a thermal transfer sheet for use in the formation of images by means of a thermal head.
Various thermal transfer sheets comprising a substrate sheet, formed of a plastic film such as a polyester resin, and, provided on the substrate sheet, a hot-melt wax layer with a pigment or a dye dispersed therein or a thermally dye sublimable transfer layer comprising a sublimable dye incorporated into a binder have hitherto been widely utilized.
However, when the thermal transfer sheet comprising a substrate sheet, formed of a plastic film, and a hot-melt wax layer or a thermally dye sublimable transfer layer provided on the substrate sheet is utilized to perform thermal printing by means of a thermal head from the substrate sheet side of the thermal transfer sheet, thermal printing under high energy necessary for realizing satisfactory print density causes the so-called xe2x80x9cstickingxe2x80x9d which is a phenomenon such that the substrate sheet is heat fused to the thermal head.
In order to solve this problem, thermal transfer sheets comprising a heat-resistant protective layer provided on the surface of the substrate sheet remote from the hot-melt wax layer or the thermally dye sublimable transfer layer have been proposed. Examples of these conventional thermal transfer sheets include: one wherein a heat-resistant resin layer formed of, for example, silicone or an epoxy resin is provided (Japanese Patent Laid-Open No. 7467/1980); and one wherein a layer comprising a lubricating inorganic pigment incorporated into a heat-resistant resin is provided (Japanese Patent Laid-Open No. 155794/1981).
Even in the case of the substrate sheets provided with a heat-resistant protective layer, however, the expansion of the range of objects used or an increase in printing speed has posed a problem that the heat resistance is unsatisfactory for realizing smooth travel of the thermal head. Further, the provision of a primer layer in many cases deteriorates heat resisting properties of the protective layer even when the protective layer per se has high heat resistance. For this reason, the application of a high temperature and a high pressure of the thermal head to the thermal transfer sheet being traveled during thermal printing sometimes makes it impossible for the thermal transfer sheet to travel and, in significant cases, causes breaking of the substrate sheet from the stuck portion.
Further, since the thermal transfer sheet is actually used in a cartridge form prepared by winding a continuous ribbon of the thermal transfer sheet, the cartridge is stored as stock or for distribution in such a state that the hot-melt wax layer with a pigment or a dye dispersed therein or a thermally dye sublimable transfer layer comprising a sublimable dye incorporated into a binder is in contact with the heat-resistant protective layer in a front-to-back relationship. In particular, in storage under high temperature conditions or storage for a long period of time, the so-called xe2x80x9ckickback phenomenonxe2x80x9d occurs wherein the dye contained in the hot-melt wax layer or the dye contained in the thermally dye sublimable transfer layer is transferred onto the heat-resistant protective layer in contact with the hot-melt wax layer or the thermally dye sublimable transfer layer. Upon rewinding, disadvantageously, the dye, which has been transferred onto the heat-resistant protective layer side, is retransferred to the dye layer side and, in this case, is transferred onto a dye layer of a different hue, resulting in remarkably deteriorated image quality.
Accordingly, it is an object of the present invention to solve the above problems of the prior art and to provide a thermal transfer sheet that is provided with a heat-resistant protective layer comprising two layers, i.e., a primer layer and a heat-resistant slip layer, provided on a substrate sheet, wherein the heat-resistant protective layer is less likely to cause a dye to be transferred onto the heat-resistant protective layer in the substrate sheet and, at the same time, has satisfactory heat resistance, and, during storage up to actual use, the dye contained in a hot-melt wax layer or a thermally dye sublimable transfer layer is less likely to be transferred onto the heat-resistant protective layer in contact with the hot-melt wax layer or the thermally dye sublimable transfer layer, and, in performing thermal printing by means of an actual thermal head, even thermal printing under high energy necessary for providing satisfactory print density does not cause heat fusing between the thermal head and the thermal transfer sheet and, thus, it is possible to prevent the inhibition of the travel of the thermal transfer sheet or the breaking of the thermal transfer sheet during travel.
The above object of the present invention can be attained by a thermal transfer sheet comprising:
a substrate sheet;
a thermally transferable ink layer provided on one surface of the substrate sheet; and
provided on the other surface of the substrate sheet, a heat-resistant protective layer comprising a primer layer and a heat-resistant slip layer provided in that order,
said primer layer being formed of a composition comprising a synthetic resin which has a storage modulus Gxe2x80x2 of not less than 106 Pa at 40xc2x0 C. and a loss modulus Gxe2x80x3 of not less than 104 Pa at 120xc2x0 C.